1. Field of the Invention
The present invention relates to a signal demodulation device used in a video floppy disk reproducing device.
2. Description of the Related Art
A typical conventional still video camera has heretofore been arranged such that the image of an object is photoelectrically converted by means of an image pickup device to catch hold of the object image in the form of a luminance signal (Y) and a color signal (C). These signals are combined together by frequency modulation and recorded on a magnetic disk in the form of a multiplex image signal. According to a recent unified standard for still video cameras, a data signal which indicates whether a particular record is a field record or frame record and which also carries other information, for example, an ordinal number of a track on which a particular record has been stored, is recorded on the magnetic disk while being superposed upon the above-described image signal by the frequency multiplexing method.
To modulate the data signal, DPSK (differential phase shift keying) modulation is adopted. Accordingly, the data signal is subjected to DPSK modulation and recorded on the magnetic disk while being superposed upon the image signal. To reproduce the recorded data, the data signal is extracted from the frequency-modulated signal read out from the magnetic disk and subjected to DPSK demodulation after waveform shaping. Various apparatuses for reproducing the data signal have been developed (for example, as disclosed in Japanese Unexamined Patent Publication No. 62-223855).
If a foreign matter, for example, adheres to the magnetic disk, however, a drop out occurs in the DPSK-modulated data signal when a reproducing apparatus reads this data signal, and therefore, an incorrect demodulation might be carried out.